f2/testlib: support wrapping commands

It's useful to obtain a fully assembled command and then wrap it to
execute into something else, e.g. gdb, strace or fakeroot.

Change-Id: Ida5928fd001925549eab89edfbf0facada7ef7f1

tests/functional2/testlib/fixtures/command.py

@@ -149,6 +149,16 @@ class Command:
         self.argv = list(argv)
         return self
 
+    def with_wrapper(self, cmd: str | Path, *args: list[str]) -> "Command":
+        """
+        Wraps the current command in the given wrapper.
+        e.g: Command(["nix", "eval", "1+1"]).with_wrapper("strace") => Command(["strace", "nix", "eval", "1+1"])
+        """
+        self.argv = [cmd, *args, *self.argv]
+        if self.exe:
+            self.exe = None if isinstance(cmd, str) else cmd
+        return self
+
     def run(self) -> CommandResult:
         """
         Runs the configured command

tests/functional2/testlib/fixtures/test_command.py

@@ -79,3 +79,10 @@ def test_command_exec_fails_on_other_bad_exit_code(
     out_msg, err_msg = msgs
     assert out_msg == "stdout: drgn fops\n"
     assert err_msg == "stderr: "
+
+
+def test_command_wrapping(command: Callable[[list[str]], Command]):
+    cmd = command(["echo $0 -- $@", "wrapper", "hello", "world"]).with_wrapper("bash", "-c")
+    res = cmd.run().ok()
+
+    assert res.stdout_plain == "wrapper -- hello world"